Technical Field
The present invention relates to a position indicator of an electromagnetic induction system and to a capacitor suitable for use as a capacitor forming a resonance circuit of the position indicator of the electromagnetic induction system.
Description of the Related Art
As disclosed in Patent Document 1 (Japanese Patent Laid-Open No. 2002-244806), for example, a coordinate input device of an electromagnetic induction system includes: a position detecting device having a sensor formed by disposing a large number of loop coils in an X-axis direction and a Y-axis direction of coordinate axes; and a pen-shaped position indicator having a resonance circuit including a coil wound around a magnetic core and a capacitor.
The position detecting device supplies a transmission signal of a predetermined frequency to a loop coil of the sensor. The loop coil transmits the transmission signal as electromagnetic energy to the position indicator. The resonance circuit of the position indicator is configured to have a resonance frequency according to the frequency of the transmission signal. The resonance circuit stores the electromagnetic energy on the basis of electromagnetic induction action between the resonance circuit and the loop coil of the sensor. Then, the position indicator returns the electromagnetic energy stored in the resonance circuit to a loop coil of the sensor of the position detecting device.
The loop coil of the sensor detects the electromagnetic energy from the position indicator. The position detecting device detects coordinate values in the X-axis direction and the Y-axis direction on the sensor, which coordinate values are indicated by the position indicator, on the basis of the position of the loop coil that supplied the transmission signal and the position of the loop coil that detected the electromagnetic energy from the resonance circuit of the position indicator.
FIG. 17 shows a general construction of an example of a pen-shaped position indicator 1 of the electromagnetic induction system in related art. The position indicator 1 in the example of FIG. 17 includes, within a hollow cylindrical casing 11, a ferrite core 13 as a magnetic core wound with a coil 12 forming a resonance circuit and a plurality of capacitors 14, 15, 16, and 17 for resonance, the capacitors 14, 15, 16, and 17 being connected in parallel with the coil 12. FIG. 17 is a sectional view of the position indicator 1, and shows the coil 12 in a state of being wound around the ferrite core 13 for purposes of illustration.
A projecting member (pen point member) 18 is joined to an end portion on a pen point side in an axial direction of the ferrite core 13. The projecting member 18 is provided so as to project from an opening 11a on the pen point side of the casing 11 to the outside. A printed board 19 is disposed and housed within the casing 11 on an opposite end portion side in the axial direction of the ferrite core 13, which opposite end portion side is opposite from the side of the projecting member 18. The capacitors 14, 15, 16, and 17 are attached on one surface of the printed board 19.
As will be described later, resonance frequency of a resonance circuit of the position indicator 1 needs to be adjusted to be a desired value. Therefore, the capacitors 14 and 15 are formed by a trimmer capacitor of variable capacitance, and the capacitors 16 and 17 are formed by a capacitor of fixed capacitance.
In addition, in the example of FIG. 17, a side switch 20 as a switch circuit is provided on the printed board 19. This side switch 20 is turned on and off by a user pressing a pressing portion exposed from a through hole (not shown) provided in the side surface of the casing 11 to the outside of the casing 11. The side switch 20 controls the turning on and off of the connection of a predetermined capacitor of a plurality of the capacitors 14, 15, 16, and 17 to the resonance circuit. Hence, the capacitance value of the capacitors forming the resonance circuit is changed by turning on and off the side switch 20, so that the phase (resonance frequency) of an electromagnetic wave transmitted from the coil 12 of the resonance circuit to a position detecting device changes.
The position detecting device can detect a position indicated by the position indicator 1 and an operation of the side switch 20 of the position indicator 1 by detecting a change in phase (frequency) of the electromagnetic wave from the position indicator 1, which electromagnetic wave is received by a loop coil. Incidentally, the operation of turning on and off the side switch 20, which operation is detected by the position detecting device, is assigned to various functions such as an operation-indicating (operation-confirming) input, for example, in an electronic device such as a personal computer in which the position detecting device is included or to which the position detecting device is externally connected.
The inductance value of the coil 12 wound around the ferrite core 13 varies from part to part. The resonance circuit of the position indicator 1 is therefore configured such that a desired resonance frequency can be obtained by adjusting the capacitance of the capacitors connected in parallel with the coil 12. In the case of the position indicator including the side switch 20 as described above, each of the resonance frequency when the side switch 20 is off and the resonance frequency when the side switch 20 is on also needs to be adjusted.
The capacitor 14 and the capacitor 15 are therefore formed by trimmer capacitors whose capacitance can be changed by operating capacitance adjusting knobs 14a and 15a, respectively. For example, the capacitor 14 is for adjustment of the resonance frequency of the resonance circuit when the side switch 20 is off, and the capacitor 15 is for adjustment of the resonance frequency of the resonance circuit when the side switch 20 is on.
The adjustment of the resonance frequency by the trimmer capacitors 14 and 15 is final fine adjustment of the resonance frequency of the resonance circuit of the position indicator 1, and is made in a state of the trimmer capacitors 14 and 15 having been incorporated in the casing 11 of the position indicator 1. For example, the capacitance of the trimmer capacitors 14 and 15 is changed by adjusting the capacitance adjusting knobs 14a and 15a of the trimmer capacitors 14 and 15 through the through hole bored in the side surface of the casing 11 to attach the side switch 20. The resonance frequency of the resonance circuit is thus adjusted.